A token ring network is a system for connecting a plurality of stations in order to have each station able to communicate with all other stations. Each station has an input line and an output line. The concentrator with the media module connects all of the lines so that signals on the output line of one station are delivered to the input line of the next downstream station. All these stations are thus connected in a line, with the output line of the last downstream station being connected to the input line of the first station. The stations are thus connected together in a ring. In order to organize the communication on the ring, a token is passed along the ring to indicate when a station can and cannot transmit onto the ring. When a station desires to transmit, it first determines if the ring is available. If the ring is available, the station transmits a frame onto the ring. The frame contains, among other things, a destination address of the station which is to receive the transmission, and a source address which is the address of the station generating the transmission. The frame then gets passed to the next downstream station. The downstream station looks at the destination address of the frame and if the frame is not destined for that station, the station then passes the frame onto the next station. When the frame reaches the destination station, the destination station reads in the data of the frame, and then continues to pass the frame along the ring. When the frame reaches the original source station, the source station removes the frame from the ring and then makes the ring available again for transmission.
One way to implement a token ring network is with a concentrator that contains one or more media modules. The media modules contain ports, and each station connects to a port in the concentrator. The media module and the concentrator connect all the ports to form the individual stations into a token ring.
If there is a defect in any of the stations, or in the lines connecting the stations to the ports, the frames may be prevented from travelling around the ring, and the token ring network is therefore inoperable.
Prior art token ring networks have a system called beacon recovery where each station emits its own beacon frame if it has not received any frames in a predetermined period. If a station receives a beacon frame, it enters a mode known as beacon repeat. In beacon repeat, the station passes subsequent beacon frames to the downstream stations. Therefore once a defect has occurred in such a token ring network, only the station immediately downstream of the defect will be issuing new beacon frames and the remaining stations will be passively repeating these beacon frames. After another predetermined period, a station will realize that it is downstream of the defect by the fact that it has not received frames and has been generating new beacon frames for longer than the predetermined period. Also since each station is usually knowledgeable of the stations upstream and downstream, a station upstream of the defect will realize that it is upstream of the defect by noticing that the new beacon frames are originating from its downstream station. Token ring beacon recovery protocol provides mechanisms by which stations both upstream and downstream of the defect will attempt to isolate the fault. In each case the station removes itself from the ring by dropping phantom current. The station performs a lobe media test. If it passes lobe media test, it rejoins the ring. If the station fails its lobe media test, it will not rejoin the ring.
There are many time periods which must expire and which have complicated relationships. Therefore the process of beacon recovery can take a relatively long time, especially if the defect is such that all of the time periods must be exhausted before the defect can be found and remedied.
The Institute of Electronic and Electrical Engineers (IEEE) has formed a well-known standard called the IEEE 802.5 communication protocol standard which is incorporated herein by reference. This IEEE 802.5 communication standard provides a general overview and background of the well-known token ring communication topology. In this standard, time limits are set for a station-based beacon recovery as described above. Fault isolation in such station beacon recovery can be in excess of 26 seconds, which can cause network disruption and loss of network sessions for individual stations. This is a relatively long period of time in a communication and is a large burden to the properly operating stations on the network.
U.S. Pat. No. 5,361,250 describes a centralized beacon recovery process, where an entity in the concentrator monitors the token ring for presence of beacon frames and then attempts to correct the defect. This reference first determines the last stations to be added to the ring and then disconnects these stations from the ring. If the ring has not recovered, then the centralized beacon recovery on the concentrator tries to determine the stations upstream and downstream of the defect and remove those stations from the ring. If this is not successful, this reference then removes all stations from the ring and gradually rebuilds the ring, one station at a time, until the defect is located.
Such a centralized beacon recovery system tends to be slow because a single Media Access Control (MAC) entity is used to isolate faults. Also, this entity must keep extensive information with regard to the individual ports and addresses of each station in the ring. As one can see, as the number of stations increases in a concentrator using centralized beacon recovery, the centralized beacon recovery system becomes more complicated, requires more resources and must perform more functions which slow down the recovery.
Also, many concentrators now have the ability to support a plurality of different and separate token ring networks on one concentrator, or even one media module. In such a centralized beacon recovery system, a separate centralized beacon recovery system will be needed for each token ring network. Resources must be available on the concentrator then to perform such a centralized beacon recovery system for every possible token ring network that could be formed on the concentrator. The number of possible token ring networks is especially large, especially when port-switchable modules are using the concentrator which can connect one port to any other port to form a separate token ring network. U.S. Pat. No. 5,361,250 is, however, very informative with regard to the state of the art and is therefore incorporated herein by reference.
U.S. Pat. No. 5,319,644 is also very informative with regard to the state of the art and is incorporated by reference.